1. Field of the Invention
This invention relates to a variable focal length lens, and in particular to a variable focal length lens in which the relative relation between a double refraction lens and the plane of polarization of a light beam incident on the double refraction lens is varied to thereby obtain a desired focal length.
2. Related Background Art
A change of the focal length of a variable focal length optical lens system called a zoom lens has heretofore been accomplished by moving a plurality of lens groups each comprising a plurality of single lenses in the optical lens system to thereby vary the spacing between those lens groups. In such a lens system, however, a movable mechanism for moving the lens groups has been necessary and as a result, performances such as rapid change of the focal length, compactness and low cost have not been sufficiently satisfied and thus, the advent of a lens system in which those performances are enhanced has been desired.
On the other hand, variable focal length lenses which are compact and have few or no movable portions and thus have solved the above-noted problems are proposed in U.S. Pat. No. 3,520,592 and our Japanese Laid-Open Patent Application No. 10224/1985. These patents discloses a variable focal length lens comprising a lens formed of a solid having optical anisotropy, and means for varying the direction of polarization of a light incident on the lens.
FIG. 1 of the accompanying drawings is a schematic view showing an example of the variable focal length lens proposed in Japanese Laid-Open Patent Application No. 10224/1985.
In FIG. 1, reference numeral 1 designates a polarizing plate, reference numeral 2 denotes a polarization plane rotating element, reference numeral 3 designates a double refraction lens, reference numeral 4 denotes a power source, and reference numeral 5 designates a switch.
The polarization plane rotating element 2 rotates the plane of polarization of transmitted light by the application of an electric field thereto, and comprises, for example, a Z-cut plate or the like of single crystal KH.sub.2 PO.sub.4 provided with transparent electrodes on the opposite sides thereof.
The double refraction lens 3 is formed so that, for example, the optical axis (Z-axis) of crystal is orthogonal to the principal axis of the lens, and is so disposed that the Z-axis is parallel to the plane of the drawing sheet. Accordingly, the refractive index of the double refraction lens 3 is such that the refractive index for the direction of polarization perpendicular to the principal axis of the lens and parallel to the plane of the drawing sheet is normal ray refractive index n.sub.0 and the refractive index for the direction of polarization perpendicular to the principal axis of the lens and perpendicular to the plane of the drawing sheet is abnormal ray refractive index n.sub.e.
In FIG. 1, the light linearly polarized by the polarizing plate 1 enters the double refraction lens 3 without changing its direction of polarization when the switch 5 is open, and thus, the light passing through this lens experiences the refractive index n.sub.0 and has a focal length F.sub.1. At this time, the direction of polarization of the transmitted light is the same as that of the first-mentioned incident light, that is, the direction parallel to the plane of the drawing sheet. In contrast, when the switch 5 is closed and a half wavelength voltage is being applied, the direction of polarization is changed to a direction rotated by 90.degree. relative to the incident light, i.e., a direction perpendicular to the plane of the drawing sheet, by the action of the polarization plane rotating element 2. Consequently, the light passing through the double refraction lens 3 has a focal length f.sub.2 which is determined by the refractive index n.sub.e for that direction.
The variable focal length lens thus changing the direction of polarization has an advantage that it can vary its focus over a wide range.
However, the electro-optical material used for the polarization plane rotating element 2, etc. is usually liable to be affected by temperature, humidity, etc. or the electro-optical material itself often has hysteresis and therefore, such material has suffered from a disadvantage that the direction of polarization of the light incident on the double refraction lens is fluctuated by the fluctuation of the half wavelength voltage applied to this electro-optical material and the lens performance is thus deteriorated. Not only the electro-optical crystal but also magneto-optical crystal or elastic material used for the polarization plane rotating element is liable to be subjected to disturbance and therefore, it is difficult to always rotate the plane of polarization normally.
Also, a lens selecting its focal length by rotating a double refraction lens relative to a predetermined linearly polarized light beam as disclosed in U.S. Pat. No. 3,520,592, or a lens selecting its focal length by rotating a polarizing plate or the like to change the plane of polarization of the incident light beam and direct the light to a double refraction lens has suffered from disadvantages similar to those of said lens having the polarization plane rotating element. That is, it has been difficult to accurately rotate the double refraction lens or the polarizing plate by a desired angle to thereby accomplish switching and therefore, it has been impossible to maintain the relation between the double refraction lens and the plane of polarization of the incident light beam in a desired relation and as a result, it has been impossible to accomplish accurate control of the focal length.